在asynctask Android中添加数据库连接

Question

我试图在我的asynctask中添加一个数据库连接。这是我正在处理的代码:

DailySync.class:

DownloadSupplierMaster objAsyncTask;
// TODO get all checked items
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.daily_synchronize);
    initControls();
    objAsyncTask = new DownloadSupplierMaster(this);     
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.dailysync_menu, menu);
        final MenuItem itemSync;
        itemSync = menu.findItem(R.id.sync);

        itemSync.setOnMenuItemClickListener(new OnMenuItemClickListener() {
            @Override
            public boolean onMenuItemClick(MenuItem item) {

                StringBuffer responseText = new StringBuffer();
                responseText.append("Selected modules are...n");
                ArrayList<SyncDataItems> itemList = dataAdapter.itemList;
                for (int i = 0; i < itemList.size(); i++) {
                 SyncDataItems itemName = itemList.get(i);
                 if (itemName.isSelected()) {
                  responseText.append("n" + itemName.getItem() );
                 }
                }

                objAsyncTask.execute();

                return true;
            }
        });

        return super.onCreateOptionsMenu(menu);
}

@Override
public void onTaskComplete(String result) {

}

DownloadTask.class:

public class DownloadTask extends AsyncTask<Void, Void, Void> {
private Activity activity;
private AsyncTaskListener callback;
public DownloadSupplierMaster(Activity act) {
 this.activity = act;
 this.callback = (AsyncTaskListener)act;
}
@Override
protected void onPreExecute() {
        super.onPreExecute();
}
protected Void doInBackground(Void...params) {
    arraylist = new ArrayList<HashMap<String, String>>();
    // Retrieve JSON Objects from the given website URL in JSONfunctions.class
    String result = JSONFunctions.getJSONfromURL(URL);
    try {
        JSONArray jr = new JSONArray(result);
        HashMap<String, String> map = new HashMap<String, String>();
        String[] keys = { SupplierMaster.TAG_SUPPLIERCODE, SupplierMaster.TAG_SUPPLIERNAME, 
                SupplierMaster.TAG_SUPPLIERGROUP, SupplierMaster.TAG_SUPPLIERTYPE, 
                SupplierMaster.TAG_SUPPLIERADDRESS, SupplierMaster.TAG_SUPPLIERADDRESSALT, SupplierMaster.TAG_PHONE1, 
                SupplierMaster.TAG_PHONE2, SupplierMaster.TAG_FAX,
                SupplierMaster.TAG_EMAIL, SupplierMaster.TAG_WEBSITE, 
                SupplierMaster.TAG_EXPENSEACCOUNT, SupplierMaster.TAG_CONTACTPERSON,
                SupplierMaster.TAG_CREDITLIMIT, SupplierMaster.TAG_TOTALCREDIT, SupplierMaster.TAG_TIN,
                SupplierMaster.TAG_TAXCODE, SupplierMaster.TAG_TERMS };
        for(int i=0;i<jr.length();i++) {    
            jb = (JSONObject)jr.get(i);
            for (String key : keys) {
                map.put(key, jb.getString(key));
            }
            arraylist.add(map);
            String suppliercode = jb.getString(SupplierMaster.TAG_SUPPLIERCODE);
            String suppliername = jb.getString(SupplierMaster.TAG_SUPPLIERNAME);
            String suppliergroup = jb.getString(SupplierMaster.TAG_SUPPLIERGROUP);
            String suppliertype = jb.getString(SupplierMaster.TAG_SUPPLIERTYPE);
            String supplieraddress = jb.getString(SupplierMaster.TAG_SUPPLIERADDRESS);
            String supplieraddressalt = jb.getString(SupplierMaster.TAG_SUPPLIERADDRESSALT);
            String phone1 = jb.getString(SupplierMaster.TAG_PHONE1);
            String phone2 = jb.getString(SupplierMaster.TAG_PHONE2);
            String fax = jb.getString(SupplierMaster.TAG_FAX);
            String email = jb.getString(SupplierMaster.TAG_EMAIL);
            String website = jb.getString(SupplierMaster.TAG_WEBSITE);
            String expenseaccount = jb.getString(SupplierMaster.TAG_EXPENSEACCOUNT);
            String contactperson = jb.getString(SupplierMaster.TAG_CONTACTPERSON);
            String creditlimit = jb.getString(SupplierMaster.TAG_CREDITLIMIT);
            String totalcredit = jb.getString(SupplierMaster.TAG_TOTALCREDIT);
            String tin = jb.getString(SupplierMaster.TAG_TIN);
            String taxcode = jb.getString(SupplierMaster.TAG_TAXCODE);
            String terms = jb.getString(SupplierMaster.TAG_TERMS);
            context.dbConnect();   // I'M GETTING A NULLPOINTEREXCEPTION IN THIS LINE!!!!!!!!!!!!
            dbHelper.insertORReplaceToSUPPLIERCARD(suppliercode, suppliername, suppliergroup, 
                    suppliertype, supplieraddress, supplieraddressalt, phone1, 
                    phone2, fax, email, website, expenseaccount, contactperson, 
                    creditlimit, totalcredit, tin, taxcode, terms);
            dbHelper.close();
        }


    } catch (JSONException e) {
        Log.e("Error", e.getMessage());
        e.printStackTrace();
    }
    return null;
} 
@Override
protected void onPostExecute(Void args) {
    callback.onTaskComplete("Test");
}

AysncTaskListener.class:

public interface AsyncTaskListener {
     public void onTaskComplete(String result);
}

问题是，我得到一个nullpointerexception在context.dbConnect();

你认为问题是什么?什么好主意吗?我很感激你的帮助。谢谢。

Answer 1

这个上下文对象是从哪里来的?你能确认它有值吗?

从设计的角度来看，我建议创建一个数据库助手类，这是一个单例，然后你可以从AsyncTask访问，让它处理整个应用程序的数据库交互，因为这将更容易处理这些类型的异常，以及使它非常容易维护所有数据库相关的代码为您的应用程序